Conventionally, a system in which a direct-current (DC) power is boosted by a boost converter, the boosted DC power is converter into an alternating-current (AC) power by an inverter, and the converted AC power is supplied to an AC motor is employed, for example, in a hybrid vehicle. Some of such systems includes two boost converters in parallel with an inverter. For example, in a system disclosed in JP 5446054 B2 (corresponding to US 2012/0074919 A1), a boost converter connected with a fuel cell and a boost converter connected with a battery are disposed in parallel with an inverter.
If an output side includes two boost converters which are parallel to each other, like the system disclosed in JP 5446054 B, the two boost converters share a high voltage line. Thus, if switching timings of the two boost converters overlap each other, surge voltages of the two boost converters are superimposed and the surge voltage becomes large. As a result, a voltage applied to switching elements may exceed a breakdown voltage. Especially, in a case where the two boost converters are formed as one unit, a wiring connecting the two boost converters becomes short. Thus, when the surge voltages of the two boost converters are superimposed, the surge voltage is likely to be large. The surge voltage of each of the boost converters is restricted when a switching speed is reduced, however a switching loss increases.